1. Technical Field
The present disclosure relates generally to spectrometers, and more specifically to optical multi-pass cells for use in long path-length spectrometers.
2. Background Information
Long path-length spectrometers may be used to make highly sensitive and precise measurements of a wide variety of gaseous molecules. In a basic configuration, a long path length spectrometer includes a light source (e.g., a laser), a multi-pass cell and a detector. The light source directs a beam into the multi-pass cell, in which a gaseous sample is disposed. The beam is repeatedly reflected within the multi-pass cell, where it may interact with the gaseous sample and be partially absorbed. By repeatedly reflecting the beam, the multi-pass cell increases the optical path length through the gaseous sample, thereby increasing absorption. A remaining portion of the beam emerges from the multi-pass cell and is detected by the light detector. A computer system may control the light source to change the wavelength or other characteristics of the beam, and receive and analyze signals from the light detector. To measure an absorption line for a gaseous molecule, the computer system may cause the light source to linearly scan over the line, and use the light detector to observe the direct absorption shape, or it may cause the light source to sinusoidally modulate, and analyze the signal from the light detector in terms of harmonic content. In practice, the configuration of long path-length spectrometers tends to be more complicated than this basic example, including other features such as multiple light sources (e.g., lasers), a reference cell, a power-nominalization path, etc. However, the general principles are similar.
For some measurement problems, for example where concentrations of the gaseous molecule are low and/or line strength is weak, the use of a multi-pass cell may be key to achieving desired detection limits. A variety of different types of multi-pass cells have been deployed over the years, including White cells, Herriott cells, and Astigmatic Herriott cells, among others. While the specific operation of these cells differs, they each produce a series of reflections between opposing mirrors that may be characterized as a spot pattern on the mirrors (herein sometime referred to simply as the “pattern”). In each of these types of multi-pass cells, a beam enters the multi-pass cell, circulates about the cell for a definite number of reflections according to the pattern, and then exits the cell. The mirrors may serve to repeatedly refocus the beam to keep it from spreading indefinitely.
White cells were the first multi-pass cells to be widely used in spectroscopy. FIG. 1 is diagram depicting an example White cell 100. The example White cell 100 includes a concave front mirror 110, and a concave split back mirror 120 consisting of two halves that are disposed at an inward tilt. The front mirror 110 and split back mirror 120 generally have the same radius of curvature. A beam 150 from a light source (e.g., a laser) (not shown) is directed onto a back mirror half 140, and repeatedly reflected between the back mirror 120 and the front mirror 110 according to a known pattern (the “White cell pattern”), before eventually exiting the White cell 100. The tilt of the back mirror halves 130, 140 may be used to adjust the number of reflections. The White cell pattern typically includes a double row of tightly focused spots on the front mirror and two sets of large overlapping spots on the back mirror.
The Herriott cell is a newer, somewhat simpler design than the White cell, that does not require a split mirror. FIG. 2 is a diagram depicting an example Herriott cell 200. The example Herriott cell 200 includes a concave front mirror 210, and a concave back mirror 220. A beam 230 from the light source (not shown) enters the Herriott cell 200 through a coupling hole 240 in the front mirror 210 at an angle, and repeatedly reflects between the back mirror 220 and the front mirror 210 according to a known pattern (the “Herriott cell pattern”), before eventually exiting the through the coupling hole 240. Typically the beam circulates with a fixed angular advance per reflection, such that the Herriott cell pattern includes an elliptical (or sometimes circular) series of spots on the front mirror 210 and back mirrors 220. The number of reflections may be changed by changing the spacing between the front mirror 210 and the back mirror 220.
In one variant of a Herriott cell, referred to as an astigmatic Herriott cell, the front mirror and the back mirror each have two different radii of curvature (e.g., have a toric surface). The astigmatic Herriott cell produces a pattern of spots which nearly fills in the area of circular mirrors (essentially a Lissajuos pattern). The number of reflections may be changed by a combination of mirror twist and spacing between the front mirror and back mirror.
While existing multi-pass cells, such as White cells, Herriott cells and Astigmatic Herriott cells, have been used successfully to conduct innumerable measurements, there are areas in which their performance may be further improved. For example, some existing multi-pass cells (e.g., astigmatic Herriott cells) utilize mirrors whose surfaces are complex and thereby expensive to produce. Similarly, some existing multi-pass cells have lower than desired optical throughput, while others may have patterns that do not fully fill the available surface area of the mirrors, resulting an inefficient use of the cell's volume. Further, some existing multi-pass cells do not provide a simple mechanism for adjusting the number of reflections.
Accordingly, there is a need for an improved multi-pass cell for long path-length spectrometers that may address some or all of these shortcomings.